Values

JavaScript recognizes the following types of values:

This relatively small set of types of values, or data types, enables you to perform useful functions with your applications. There is no explicit distinction between integer and real-valued numbers. Nor is there an explicit date data type in JavaScript. However, you can use the Date object and its methods to handle dates.

Objects and functions are the other fundamental elements in the language. You can think of objects as named containers for values, and functions as procedures that your application can perform.

Data type conversion

JavaScript is a dynamically typed language. That means you do not have to specify the data type of a variable when you declare it, and data types are converted automatically as needed during script execution. So, for example, you could define a variable as follows:

var answer = 42;

And later, you could assign the same variable a string value, for example:

answer = "Thanks for all the fish...";

Because JavaScript is dynamically typed, this assignment does not cause an error message.

In expressions involving numeric and string values with the + operator, JavaScript converts numeric values to strings. For example, consider the following statements:

x = "The answer is " + 42 // "The answer is 42"
y = 42 + " is the answer" // "42 is the answer"

In statements involving other operators, JavaScript does not convert numeric values to strings. For example:

"37" - 7 // 30
"37" + 7 // "377"

Converting strings to numbers

In the case that a value representing a number is in memory as a string, there are methods for conversion.

parseInt() and parseFloat()

See: parseInt() and parseFloat() pages.

parseInt will only return whole numbers, so its use is diminished for decimals. Additionally, a best practice for parseInt is to always include the radix parameter.

Plus operator

An alternative method of retrieving a number from a string is with the + operator.

"1.1" + "1.1" = "1.11.1"
(+"1.1") + (+"1.1") = 2.2   
// Note: the parentheses are added for clarity, not required.

Variables

You use variables as symbolic names for values in your application. The names of variables, called identifiers, conform to certain rules.

A JavaScript identifier must start with a letter, underscore (_), or dollar sign ($); subsequent characters can also be digits (0-9). Because JavaScript is case sensitive, letters include the characters "A" through "Z" (uppercase) and the characters "a" through "z" (lowercase).

Starting with JavaScript 1.5, you can use ISO 8859-1 or Unicode letters such as å and ü in identifiers. You can also use the \uXXXX {{anch("Unicode escape sequences")}} as characters in identifiers.

Some examples of legal names are Number_hits, temp99, and _name.

Declaring variables

You can declare a variable in two ways:

• With the keyword var. For example, var x = 42. This syntax can be used to declare both local and global variables.
• By simply assigning it a value. For example, x = 42. This always declares a global variable and generates a strict JavaScript warning. You shouldn't use this variant.

Variable scope

When you declare a variable outside of any function, it is called a global variable, because it is available to any other code in the current document. When you declare a variable within a function, it is called a local variable, because it is available only within that function.

JavaScript does not have block statement scope; rather, it will be local to the code that the block resides within. For example the following code will log 5, because the scope of x is the function (or global context) within which x is declared, not the block, which in this case is an if statement.

if (true) {
  var x = 5;
}
//statements

Because of hoisting, all var statements in a function should be placed as near to the top of the function as possible. This best practice increases the clarity of the code.

Constants

You can create a read-only, named constant with the const keyword. The syntax of a constant identifier is the same as for a variable identifier: it must start with a letter or underscore and can contain alphabetic, numeric, or underscore characters.

const prefix = '212';

A constant cannot change value through assignment or be re-declared while the script is running.

The scope rules for constants are the same as those for variables, except that the const keyword is always required, even for global constants. If the keyword is omitted, the identifier is assumed to represent a variable.

You cannot declare a constant with the same name as a function or variable in the same scope.

Literals

You use literals to represent values in JavaScript. These are fixed values, not variables, that you literally provide in your script. This section describes the following types of literals:

Array literals

An array literal is a list of zero or more expressions, each of which represents an array element, enclosed in square brackets ([]). When you create an array using an array literal, it is initialized with the specified values as its elements, and its length is set to the number of arguments specified.

The following example creates the coffees array with three elements and a length of three:

var coffees = ["French Roast", "Colombian", "Kona"];

Note An array literal is a type of object initializer. See Using Object Initializers.

If an array is created using a literal in a top-level script, JavaScript interprets the array each time it evaluates the expression containing the array literal. In addition, a literal used in a function is created each time the function is called.

Array literals are also Array objects. See Array Object for details on Array objects.

Extra commas in array literals

You do not have to specify all elements in an array literal. If you put two commas in a row, the array is created with undefined for the unspecified elements. The following example creates the fish array:

var fish = ["Lion", , "Angel"];

This array has two elements with values and one empty element (fish[0] is "Lion", fish[1] is undefined, and fish[2] is "Angel").

If you include a trailing comma at the end of the list of elements, the comma is ignored. In the following example, the length of the array is three. There is no myList[3]. All other commas in the list indicate a new element. (Note trailing commas can create errors in older browser versions and it is a best practice to remove them)

var myList = ['home', , 'school', ];

In the following example, the length of the array is four, and myList[0] and myList[2] are missing.

var myList = [ , 'home', , 'school'];

In the following example, the length of the array is four, and myList[1] and myList[3] are missing. Only the last comma is ignored.

var myList = ['home', , 'school', , ];

Understanding the behavior of extra commas is important to understanding JavaScript as a language, however when writing your own code: explicitly declaring the missing elements as undefined will increase your code's clarity and maintainability.

Boolean literals

The Boolean type has two literal values: true and false.

Do not confuse the primitive Boolean values true and false with the true and false values of the Boolean object. The Boolean object is a wrapper around the primitive Boolean data type. See Boolean Object for more information.

Integers

Integers can be expressed in decimal (base 10), hexadecimal (base 16), and octal (base 8).

• Decimal integer literal consists of a sequence of digits without a leading 0 (zero).
• Leading 0 (zero) on an integer literal indicates it is in octal. Octal integers can include only the digits 0-7.
• Leading 0x (or 0X) indicates hexadecimal. Hexadecimal integers can include digits (0-9) and the letters a-f and A-F.

Octal integer literals are deprecated and have been removed from the ECMA-262, Edition 3 standard (in strict mode). JavaScript 1.5 still supports them for backward compatibility.

Some examples of integer literals are:

0, 117 and -345 (decimal, base 10)
015, 0001 and -077 (octal, base 8) 
0x1123, 0x00111 and -0xF1A7 (hexadecimal, "hex" or base 16)

Floating-point literals

A floating-point literal can have the following parts:

• A decimal integer which can be signed (preceded by "+" or "-"),
• A decimal point ("."),
• A fraction (another decimal number),
• An exponent.

The exponent part is an "e" or "E" followed by an integer, which can be signed (preceded by "+" or "-"). A floating-point literal must have at least one digit and either a decimal point or "e" (or "E").

Some examples of floating-point literals are 3.1415, -3.1E12, .1e12, and 2E-12.

More succinctly, the syntax is:

[digits][.digits][(E|e)[(+|-)]digits]

For example:

3.14
2345.789
.3333333333333333333

Object literals

An object literal is a list of zero or more pairs of property names and associated values of an object, enclosed in curly braces ({}). You should not use an object literal at the beginning of a statement. This will lead to an error or not behave as you expect, because the { will be interpreted as the beginning of a block.

String literals

A string literal is zero or more characters enclosed in double (") or single (') quotation marks. A string must be delimited by quotation marks of the same type; that is, either both single quotation marks or both double quotation marks. The following are examples of string literals:

• "foo"
• 'bar'
• "1234"
• "one line \n another line"
• "John's cat"

You can call any of the methods of the String object on a string literal value—JavaScript automatically converts the string literal to a temporary String object, calls the method, then discards the temporary String object. You can also use the String.length property with a string literal:

"John's cat".length

You should use string literals unless you specifically need to use a String object. See String Object for details on String objects.

Using special characters in strings

In addition to ordinary characters, you can also include special characters in strings, as shown in the following example.

"one line \n another line"

The following table lists the special characters that you can use in JavaScript strings.

Escaping characters

For characters not listed in Table 2.1, a preceding backslash is ignored, but this usage is deprecated and should be avoided.

You can insert a quotation mark inside a string by preceding it with a backslash. This is known as escaping the quotation mark.

The result of this would be:

He read "The Cremation of Sam McGee" by R.W. Service.

To include a literal backslash inside a string, you must escape the backslash character. For example, to assign the file path c:\temp to a string, use the following:

var home = "c:\\temp";

You can also escape line breaks by preceding them with backslash. The backslash and line break are both removed from the value of the string.

var str = "this string \
is broken \
across multiple\
lines."

Although JavaScript does not have "heredoc" syntax, you can get close by adding a linebreak escape and an escaped linebreak at the end of each line:

var poem = 
"Roses are red,\n\
Violets are blue.\n\
I'm schizophrenic,\n\
And so am I."